For the first time, we developed an on-the-fly method OFIT to measure the interface trap density NIT without recovery during measurement. The OFIT produces the most reliable experimental data of the interface trap generation dynamics under stress and therefore provides a solid ground to check various modeling work. The slope n of tn time evolution of DeltaNIT under stress is temperature dependent, supporting dispersive Hydrogen transport in the oxide. Comparing OFIT data with the data measured by ultra-fast pulsed Vth measurement, we successfully decompose the NBTI DeltaVTH into interface trap component DeltaVTH IT and oxide charge component DeltaVTH OX quantitatively for the p-MOSFETs with SiON gate dielectric.